Production of tightly adherent coatings on metal articles



rnonuc'noN F TIGHTLY ADHERENT COATINGS 0N METAL ARTICLES Fritz Singer, Starnberg, Upper Bavaria, Germany No Drawing. Appllcation'lune 14, 1955 Serial No. 515,553

" 25 Claims. (Cl.1486.24)

This invention relates to the production of tightly adherent coatings on metal articles. The invention is es- United States Patent 0 2376,15! Patented Mar. 3, 1959 "Ice.

. gen sulfide.

pecially useful in the production of coatings on ferrous surfaces, such as on iron and steel articles. The coated articles are well adapted for cold working or plastic deformation.

This application is a continuation-in-part of my copending application, Serial No. 357,351, filed May 25,

1953, now abandoned, and the disclosure of the latterv application is incorporated herein and made a part hereof by reference as fully as if it were set forth herein in its entirety. I

In cold working metals, it is necessary to provide an intermediate layer between the metal to be worked and the tool or die, as otherwise, the tool would bite into the metal, resulting in damage to the tool and the metal surface to be worked. Numerous materials have been pro-,

posed for this purpose.

It has been found that a protective layer on the metal surface is advantageously a layer of the sulfurized metal, formed on the surface of an article by reaction with the metal. A number of methods for producing a coating or layer of sulfurized metal have been proposed. Thus, ferrous surfaces have been coated by reaction with a heated aqueous acidic solution of sulfur, sulfur dioxide and polythionic acids, prepared by the decomposition of sodium polythionate by acids. It has been proposed to provide an alkaline layer of sodium sulfite or polythionate on an "article and heat it in an annealing furnace. Coats have been produced by reacting a strongly alkaline solution of sodium thiosulfate or sodium sulfite with the metal at elevated temperatures. Coatings of sulfurized metal have also been obtained by reaction of the metal surface with alkali metal thiosulfates, sulfites and polythionates in the presenceof activators or accelerators, such as halides and acidsf Such prior methods, however, suffered from disadvantages in commercial application. For example, acids or alkalis need be employed, operation at elevated temperatures is necessary, or a plurality of ingredients is required.

It is, therefore, an object of the invention to provide a method for forming a tightly adherent coating on the surface of a metal article which overcomes the disadvantages of the prior methods.

A particular object is to provide a new and improved method for producing a tightly adherent coating of sulfurized metal on the surface of an article.

Another object is to furnish a method which requires no acid or alkali, but which is performed simply by reaction with a substantially neutral sulfur compound.

An additional object is to provide a simple and reliable method for coating a metal surface which requires for its accomplishment the use of only very dilute aqueous solutions. I

A further object is to provide a method which can be salt.

A still further object is to furnish a method in which there is no sediment formation which contaminates the coating bath. I

A still further object is to provide a method which requires no analytical control.

Another important object is to provide an improvement in the process of cold working a metal article.

These and other objects will appear throughout the specification. p A new method in accordance with the invention constitutes contacting the surface of a metal article with water and an ammonium salt of a reducing oxygen- .containing acid of sulfur until a tightly adherent coating of sulfurized metal is formed on the surface. The invention is especially useful for coating ferrous surfaces, such as on iron and steel articles. The coated article is very advantageously adapted for cold working.

The metal surface is coated with an ammonium salt of any reducing oxygen-containing acid of sulfur, such as ammonium sulfite (NHQ SO ammonium thiosulfate (NH S O ammonium polythionates (NI-10 8 0,, where x is 2 to 5, or similar compounds. Inorganic compounds are preferred, and, for reasons of economy, ammonium sulfite is further preferred. The coat may be formed by contacting the article with 'an aqueous solution of'the ammonium salt, preferably in a bath. Formation of the coat can be accomplished in other ways, such as by dipping an article in such a solution, to provide a film of solution on the metal surface, and allowing the filmed surface to stand in the open air.

The coating composition is preferably substantially neutral, i. e., with no additional inclusion of acid or alkaline substances. The ammonium salt is preferably the sole reactant, with no further sulfurizing agents or other reactants and with no activators or accelerators present.

The ammonium salt solution may be very dilute, for example, from 215% by weight, or greater, the concentration depending upon the procedure employed. Thus, when a metal article is coated in a bath, the solution, especially an ammonium sulfite solution or a solution of a similar ammonium salt, preferably contains less than about 5% of the ammonium salt. Somewhat higher concentrations may be used but require an increase of reaction time and preferably the use of elevated temperature's. The higher reactivity of very diluted aqueous solutions of ammonium sulfite is apparently due to the fact that such solutions of the ammonium salts are dissociated to a higher degree into ammonia and sulfurous acid by hydrolysis than more concentrated solutions thereof.

A- more concentrated ammonium salt solution may be employed when the coating is effected by dipping the metal article in the solution, draining the excess liquid to leave a film of the solution on the surface, and allowing the filmed article to stand in air. This method is especially advantageous when using ammonium thiosulfate, ammonium polythionate, or a similar ammonium As indicaed above, hydrolysis into sulfurousacid apparently plays an important part in the success of the operation. This property is peculiar to the ammonium salts, whereas the alkali metal salts and the like form metal hydroxide which prevents the formation of the sulfurized metal coat. For the same reason, solutions of greater than slight alkalinity, i. e., relatively strongly basic solutions, are to be avoided. Likewise, relatively strongly acidic solutions dissolve the compounds of iron and sulfur and are to be avoided.

It is to be noted that ammonium salts of organic acids of sulfur which dissociate into ammonia and sulfurous acid, or ammonium sulfite, may be used. For example, the ammonium salt of a monoalkylether of sulfurous acid can beemployed. However, their expense renders their use of much less practical importance than the inorganic salts, and economy is a primary object of the invention.

Reaction takes place at atmospheric temperatures. When the coat is formed in a bath, the operation is satisfactorily carried out at room temperatures, i. e., about 15-35 C. Formation of the coat can be accelerated, if desired, by heating the solution, for example, up to the boiling point." It is-preferred, however, to carry out the reaction in the range of from room temperature to about 50 C'., to avoid loss of ammonia, as will appear, and for most desirable commercial operation.

' When coating a ferrous article in a bath at room tem perature, with ammonium sulfite, for example, the bright surface of the metal becomes dull in a short period of time,-a few minutes. A greyish-black tightly adhering film is gradually produced, the time depending on the desired thickness of the coating. For many cold working operations, 1 to 2 hours at room temperatures is sufficient. The time can be shortened by heating the solution, especially for thicker coats; for example, the coat is completely formed by contact with a solution of about 50 C. in 30 minutes, the time depending, however, on the thickness desired. A fine crystalline coat or film adheres tightly to the surface of the article, which appears to consist of basic iron sulfite and interspersed iron sulfide. With another reagent, the composition of the coat may vary.

When the articles are dipped in a solution, drained and allowed to stand in air, the black or greyish-black adherent coat forms after a number of hours. For example, with ammonium thiosulfate or ammonium polythionate, the coat forms in about 1 to 2 hours under average atmospheric conditions. Using such agents, the coatapparently consists mainly of iron sulfide.

Once the sulfurized metal coat has formed, it is removed from exposure to humid air, to avoid the transformation of the sulfide coating into ferric hydroxide by the further action of the oxygen of the air which takes place in the presence of moisture. The oxidation may bev prevented by storing the coated articles in dry air or by coating them with a lubricant used for subsequent cold working. An antioxidant, such as hydroquinone,

may also be included in the treating solution, to prevent 5 oxidation of the sulfurized metal coat as well as of the ammonium salt.

In this connection, it is inadvisable to spray the articles with aqueous solutions of the ammonium salts in an oxygen-containing atmosphere, as the ammonium salts are extremely susceptible to oxidation, which would produce ineffective ammonium salts such as ammonium sulfate- The reactions proceed with the evolution of ammonia, which remains in solution in the bath treatment. The resulting alkalinity is insufiicient to materially affect the reaction, and the pH is substantially neutral for the purposes of the process. Preferably, ammonium sulfite is regenerated in situ by introducingsulfur dioxide to keep the pH at about 6.5-7. The pH of the solution is thus maintained at substantially neutral, which is most favorable for the formation of the coating. There is no requirement for analytical control, and the pH can be checked with indicator paper.

The conservation of ammonia in the bath t eatmen 4 a and the simplicity of regeneration of ammonium sulfite constitute advantages over the dipping and exposure to air, as ammonia is lost in the latter method. An advantage of the latter method, particularly with ammonium thiosulfate, ammonium polythionate or the like, as preferred, isthat the greater content of metal sulfide in the coating increases its lubricating properties. Also, the operation and equipment are very simple.

- In applying the ammonium salt solution as a film on the surface of an article, an adhesive agent, such as methyl cellulose or cellulose glycolate, may be included in the composition, in order to insure uniform distribution of the ammonium salt on the metal surface and adhesion thereto. Wetting agents are preferably not included, as such substances retard the action of the ammonium salt.

The coated articles thus produced are then suited for cold working operations, applying such lubricants as are used in cold working phosphate and otherwise coated metal articles. The process is especially useful in cold drawing and cold extrusion of metals, especially in wire, profile and tube drawing.

The following examples are furnished to illustrate the invention, but it is to be understood that they are only illustrative and that the invention is not limited thereto nor to the specific ingredients, proportions, and conditions set forth therein.

Example I Iron or steel articles are immersed in a solution of 3.5 parts by weight of ammonium sulfite in 96.5 parts by weight of water. The solution is at room temperature, in this case, about C.

The reaction starts with the bright surface becoming dull in a few minutes. A greyish-black tightly adhering coating gradually forms. A strongly adherent fine-crystalline coat or film is produced uniformly over the surfaces of the articles in one or more hours, depending on the reactivity of the metal and the desired thickness 4 of the coat. The coating isvsimilar to the phosphate coating in common use, its crystal grain being much finer than that of the latter. The coating is generally resistant to atmospheric oxidation.

Thicker coatings are obtained and reaction time shortened by heating the solution up to C., at which temperature the formation of the coating requires about 30 minutes. 1

In a similar manner, the articles are provided with a sulfurized iron coating by immersion in a solution of 2-10% by weight of an ammonium polythionate. The coating is black and consists mainly of iron sulfide.

The articles are then cold worked, drawing them through a die. The coatings function very effectively to protect the surfaces of the articles and the tool.

5 Example II 5 parts by weight of ammonium thiosulfate are dissolved in 95 parts by weight of water. At a temperature of 20 to 25 C., iron or steel articles are immersed in this solution.

After about 3 hours standing, a thick black coating is produced which consists of a greyish-brown layer strongly adherent to the iron surface superposed with a layer of less strongly adherent black iron sulfide.

The coated articles are then immediately cold worked,

the superposed layer of iron sulfide acting as a lubricant or being immersed in an oil-in-water emulsion or another lubricant and then cold worked.

While the coating according to Example I is fairly resistant to corrosion, the black coating is liable to be oxidized in a humid atmosphere due to its consisting mainly of iron sulfide. If stored, the coating should be protected from the action of humid atmosphere.

' Example III Iron or steel articles are degreased with a fatty alcohol sulfonate, washed and then dipped in a solution of 10 parts by weight of ammonium thiosulfate in 90 parts by weight of water. The articles are allowed to drain and stand in the open air.

After standing for about one to two hours in air, a black or greyish-black tightly adherent coat forms on the articles. It appears that in the course of the reaction, ammonia is evolved, leaving sulfurous acid and sulfur on the surfaces which form the coat.

It is observed that if the articles are not subsequently protected from moisture by storing under dry conditions or otherwise protecting them from moisture, the coat gradually changes to iron hydroxide.

A lubricant is applied to the articles, either at the completion of the coating formation or at the time of working, which occurrences may be the same, and the articles are ,cold worked.

The invention thus provides a very simple, economical and trouble-free method of coating metal articles, especially useful in cold working operations; The method is especially advantageous in that no acids or alkalies need be employed, and the solutions are for practical purposes substantially neutral. There is no necessity for the use of activators or accelerators. The method can be carried out entirely under atmospheric conditions. The operations are safe and clean, and no noxious or annoying fumes of sulfur compounds are evolved. The reactions proceed without, waste of reagent.

While the alkali metal saltsl of the reducing oxygencontaining acids of sulfur do not react with iron surfaces in the absence of acids or alkalies, it is a surprising fact that the ammonium salts thereof have been found to react even in the form of very dilute aqueous solutions and alreadyat room temperatures to form tightly adherent coatings. The discovery of this reactivity and its utilization in cold working operations constitute a marked advance in the art of considerablepractical importance.

The invention is hereby claimed as follows:

1. The method which comprises contacting the surface of a ferrous article with a composition consisting essentially of water and ammonium thiosulfate until a tightly adherent coating of sulfurized iron is formed on said surface. a I

2. The method which comprises contacting the surface of a ferrous article with a composition consisting essentially of water and an ammonium polythionate until a tightly adherent coating of sulfurized iron is formed on said surface.

3. The method of forming a sulfurized coating on a ferrous surface which comprises contacting the surface of the ferrous article with a solution consisting essentially of water and an ammonium salt of a reducing oxygencontaining acid of sulfur until a tightly adherent coating of sulfurized iron is formed on said surface.

4. The method of claim 3 further defined in that said ammonium salt is ammonium sulfite.

5. The method of claim 3 further defined in that said ammonium salt is ammonium thiosulfate.

6. The method of claim 3 further defined in that sai ammonium salt is ammonium polythionate.

7. The method of claim 3 further defined in that said ammonium salt is an ammonium salt of a monoalkylether of sulfurous acid.

8. The method of claim 3 further defined in that the concentration of said ammonium salt ranges from about 2% to about by weight.

9. The method of claim 3 further defined in that the concentration of said ammonium salt ranges from about 3.5% to about 10% by weight.

10. The method of claim 3 further defined in that the concentration of said ammonium salt is about 3.5% by weight.

11. A composition for the treatment of ferrous surfacesto form a sulfurized coating thereon, said composition consisting essentially of a 2 to 15% aqueous solution of an ammonium salt of a reducing oxygen-containing acid of sulfur and methyl cellulose.

12. The composition of claim 11 further defined in that said ammonium salt is ammonium thiosulfate.

"13. The composition of claim 11 further defined in that said ammonium salt is ammonium polythionate.

14. The composition of claim ll further defined in that said ammonium salt is ammonium sulfite.

15. The composition of claim 11 further defined in that the concentration of the ammonium salt in said aqueous solution ranges from about 3.5% to about 10% by weight.

16. The composition of claim 11 further defined in that the concentration of the ammonium salt in said aqueous solution is about 3.5% by weight.

17. A composition for the treatment of ferrous surfaces to form a sulfurized coating thereon, said composition consisting essentially of a 2 to 15% aqueous solution ofian ammonium salt of a reducing oxygen-containing acid of sulfur and cellulose glycolate.

18. The composition of claim 17 further defined in that the concentration of the ammonium salt in said aqueous solution ranges from about 3.5% to about 10% by weight.

19. The composition of claim 17 further defined in that the concentration of the ammonium salt in said aqueous solution is about 3.5 by weight.

20. The composition of claim 17 further defined in that said ammonium salt is ammonium thiosulfate.

21. The composition of claim 17 further defined in that-said ammonium salt is ammonium polythionate.

22. The composition of claim 17 further defined in that said ammonium salt is ammonium sulfite.

23. A ferrous metal article treated for plastic deformation, said article being provided with a tightly adherent sulfurized coating by contacting the metal with an aqueous solution consisting essentially of 2 to 15% of an ammonium salt of a reducing oxygen-containing acid of sulfur.

24. The article of claim 23 further defined in that said concentration of ammonium salt is from 3.5% to 10%.

.25. The article of claim 23 further defined in that said concentration of ammonium salt is 3.5

References Cited in the file or this patent UNITED STATES PATENTS 2,105,672 Sadtler Jan. 18, 1938 2,266,378 Farrington et al. Dec. 16, 1941 2,346,609 Riggs et al. Apr. 11, 1944 2,519,672 Lawless Aug. 22, l950 2,588,234 Henricks Mar. 4, 1952 2,639,245 Baxter May 19, i953 2,644,774 Baxter July 7, 1953 2,728,696 Singer Dec. 27, 1955 OTHER REFERENCES Hackh: Chemical Dictionary, page 49, published 

1. THE METHOD WHICH COMPRISES CONTACTING THE SURFACE OF A FERROUS ARTICLE WITH A COMPOSITION CONSISTING ESSENTIALLY OF WATER AND AMMONIUM THIOSULFATE UNTIL A TIGHTLY ADHERENT COATING OF SULFURIZED IRON IS FORMED ON SAID SURFACE. 